Diffractive Optics Athermalize IR Systems

The properties of materials employed in infrared lenses generally exhibit significant variations with changes in ambient temperature. As a result, the control of thermally induced degradation in image quality is an important issue in optical systems operating at infrared wavelengths. These include the objective lenses of thermal imagers and the optics employed in CO2 laser-machining systems.

Precision-Optical Engineering in Hitchin, Hertfordshire, England, has developed hybrid refractive-diffractive lenses for passive athermalization of the optical components of thermal imagers. They are generally in the form of a conventional aspheric lens with a blazed zone plate diffractive structure superimposed on the non-spherical surface. Development of room temperature staring-array sensors operating over the 8 to 14um range offers the potential of lightweight, low-cost and compact thermal imagers. This requires optical systems that emphasize a given level of performance with the minimum number of elements. 

The typical lens system comprises two germanium elements, the front one being an asphere in a Petzval-type arrangement. This type of lens maintains acceptable optical performance over a range of a few degrees Celsius about the ambient, though the lens may be required to perform over -20 to +40 'C. Typically, this discrepancy is accommodated by manual adjustment of the focus. Precision-Optical Engineering has developed an athermalized version of this Petzval-type lens that maintains lens performance over the full operating temperature range.

This component replaces the front germanium aspheric element with a doublet that comprises a negative germanium hybrid element and a positive spherical AMTIR or zinc Selenide component. While the lens doublet stabilizes operation over a wide temperature range, the diffractive surface controls the resulting primary longitudinal chromatic aberration, a particularly important consideration in this design. Although secondary spectrum is typically larger in hybrid lens systems than in equivalent conventional designs, the germanium AMTIR doublet results in a very small level of this aberration, and near-diffraction-limited performance can be achieved on-axis. A similar hybrid solution not employing a diffractive surface would require an additional lens material and therefore four lens elements. 

The diffractive surface contains about 25 zones over a diameter of 75 mm and can be readily manufactured with high diffraction efficiency by single-point diamond turning, according to Precision-Optical Engineering. The performance of such a lens has been assessed in the laboratory over the full operating temperature range and matches well with theoretical predictions. 

The thermal focus shift introduced into lenses by high-power CO2 laser-machining operations can also significantly degrade system performance. Precision-Optical Engineering uses a diffractive surface in this case to directly cancel the thermal defocus introduced by a conventional refractive lens. The result is a diffractive structure with many more zones (100 to 200) than are typically required in hybrid optics employed for the correction of chromatic aberrations.

Reprinted from the February 1995 issue of PHOTON lCS SPECTRA © Laurin Publishing Co., Inc.